The present invention is in the field of optics. More specifically, the invention makes use of volume holograms to convert an extended, polychromatic light source to an efficient spatially coherent, quasi-monochromatic light source.
Holograms recorded in thick media are called volume holograms. Since a thick recording medium is not generally a sufficient condition for forming a volume hologram, "thickness" is defined in terms of a quality factor Q defined as ##EQU1## WHERE .lambda. IN THE WAVELENGTH OF THE COHERENT LIGHT SOURCE USED TO CONSTRUCT THE HOLOGRAM, T.sub.O IS THE PHYSICAL THICKNESS OF THE HOLOGRAM RECORDING MEDIUM, N IS THE INDEX OF REFRACTION OF THE RECORDING MEDIUM AND D IS THE AVERAGE THREE DIMENSIONAL GRATING PERIOD. As a rule, holograms are classified as the volume type provided that Q &gt;&gt; 10. In practice, this requires a recording media that has a physical thickness on the order of (1) millimeter (mm) or greater.
The response of holograms in thick media, such as thick photographic emulsions, photopolymers, photodegradable polymers and so forth is measured in terms of diffraction efficiency, angular orientation sensitivity and wavelength sensitivity of the reconstruction. These factors are themselves a function of several hologram recording parameters, including thickness of the hologram, angles of incidence of the light beams used to construct the hologram, angle of incidence of the reconstruction (i.e., readout) beam, and exposure characteristics of the hologram medium.
The diffraction efficiency is the ratio of the intensity of the light diffracted by the hologram upon reconstruction, to the intensity of the incident light. The quality of the hologram may be judged by the amount of diffracted light contributing to the reconstruction of the original wavefront.
The angular orientation sensitivity (i.e., the sensitivity in terms of the intensity of the diffracted wave for various angular orientations between the surface of the hologram and the incident readout beam) is measured as the angular width between half-power points of the curve of diffracted wave intensity versus the readout beam angle.
The wavelength sensitivity of the reconstruction can be measured as the wavelength bandwidth between half-power points on the curve of diffracted intensity versus beam wavelength.
Construction of a hologram in a thick recording medium is typically affected by recording the interference pattern of two coherent monochromatic waves constituting the signal beam and the reference beam. The signal beam 10 and the reference beam 11 are incident on the surface of the thick recording medium 12 at arbitrary angles .theta..sub.s and .theta..sub.r, respectively, to the normal to the surface of the medium 12, as shown in FIG. 1. The two beams are oriented relative to the surface of the medium such that the interference fringe surfaces are oblique to that surface. In the regions of the recording medium where the incident signal and reference beams overlap, interference fringe planes 14 of constant intensity, and hence constant exposure, bisect the angle between the two wavefronts. These planes form a three-dimensional pseudograting of thickness t.sub.o having a period in one direction (i.e., the direction of a line perpendicular to the bisector of the angle between the two beams, in a plane containing the rays representative respectively of the two beams).
Readout is effected by illuminating the hologram (which is simply the interference pattern recorded in the thick medium) with a reconstructing beam constituting one of the two original waves used in the recording process, at the original angle of incidence of that wave. This causes the emergence from the hologram of a diffracted wavefront identical to the complex wavefront of the other original wave. An observer or detector positioned in the path of the diffracted wavefront sees a three-dimensional virtual image of the object originally illuminated by the signal beam.
It is desirable for enhancing diffraction efficiency, to obtain uniform penetration of the thick recording medium by the two recording beams (signal and reference). Uniformity of penetration is largely dependent upon the nature of the recording medium.
It has been observed that the angular orientation sensitivity of volume holograms is high when Q &gt;&gt; 10; that is, little light is diffracted from the hologram when the angle of incidence of the readout beam differs from the Bragg angle (one-half the angle between the two original recording beams) by more than a few minutes of arc.
It has also been observed that the wavelength selectivity of volume holograms is high when Q &gt;&gt; 10; that is, little light is diffracted from the hologram when the reconstructing wavelength differs from the recording wavelength by more than about 10 A.
In both cases the half-power bandwidth is known to be inversely proportional to the thickness t.sub.o of the hologram.